1. Field of the Invention
The present invention relates to ferromagnetic amorphous alloy ribbon and to a marker for use in an electronic article identification system, the marker including a plurality of rectangular strips based on an amorphous magnetostrictive material that vibrates in an alternating magnetic field mechanically at multiple resonant frequencies, whereby the magnetomechanical effect of the marker is effectively utilized for encoding and decoding purposes. The present invention is also directed to an electronic identification system utilizing such a marker.
2. Background of the Invention
Magnetostriction of a magnetic material is a phenomenon in which a dimensional change takes place upon application of an external magnetic field on the magnetic material. When the dimensional change is such that the material elongates upon its being magnetized, the material is termed “positive-magnetostrictive”. When a material is “negative-magnetostrictive”, the material shrinks upon its magnetization. Thus in either case, a magnetic material vibrates when it is in an alternating magnetic field. When a static magnetic field is applied along with the alternating field, the frequency of the mechanical vibration of the magnetic material varies with the applied static field through magneto-elastic coupling. This is commonly known as ΔE effect, which is described, for example, in “Physics of Magnetism” by S. Chikazumi (John Wiley & Sons, New York, 1964, page 435). Here E(H) stands for Young's modulus which is a function of an applied field H, and the material's vibrational or resonance frequency fr is related to E(H) throughfr(½l)[E(H)/ρ]1/2,  (1)where l is the length of the material and ρ is the mass density of the material. The magneto-elastic or magneto-mechanical effect described above is utilized in electronic article surveillance systems which were first taught in the U.S. Pat. Nos. 4,510,489 and 4,510,490 (hereinafter the '489 and '490 patents). Such surveillance systems are advantageous systems, in that they offer a combination of high detection sensitivity, high operating reliability and low operating costs.
The marker in such systems is a strip, or a plurality of strips, of known length of a ferromagnetic material, packaged with a magnetically harder ferromagnet (material with a higher coercivity) that provides a static field termed as biasing field to establish peak magneto-mechanical coupling. In accordance with embodiments of the invention, ferromagnetic marker material is an amorphous alloy ribbon, since the efficiency of magneto-mechanical coupling in the alloys is very high. The mechanical resonance frequency, fr is determined essentially by the length of the alloy ribbon and the biasing field strength, as the above Equation (1) indicates. When an interrogating signal tuned to the resonance frequency is encountered in an electronic identification system, the marker material responds with a large signal field which is detected by a receiver in the system.
Several amorphous ferromagnetic materials were considered in the U.S. Pat. No. 4,510,490 for coded identification systems based on magnetomechanical resonance described above and included amorphous Fe—Ni—Mo—B, Fe—Co—B—Si, Fe—B—Si—C and Fe—B—Si alloys. Of the alloys, a commercially available amorphous Fe—Ni—Mo—B based METGLAS®2826MB alloy was used extensively until accidental triggering, by a magnetomechanical resonance marker, of other systems based on magnetic harmonic generation/detection. This occurs because a magnetomechanical resonance marker used at that time sometimes exhibited non-linear BH characteristics, resulting in generation of higher harmonics of the exciting field frequency. To avoid this problem, sometimes called a system “pollution problem,” a series of new marker materials have been invented, examples of which are disclosed in U.S. Pat. Nos. 5,495,231, 5,539,380, 5,628,840, 5,650,023, 6,093,261 and 6,187,112. Although the new marker materials perform, on average, better than the materials utilized in the surveillance systems of the original '489 and '490 patents, somewhat better magnetomechanical performance has been found in the marker materials disclosed, for example, in U.S. Pat. No. 6,299,702 (hereinafter '702 patent). These new marker materials require complex heat-treatment processes to achieve desired magnetomechanical properties as disclosed, for example, in the '702 patent. Clearly, a new magnetomechanical marker material is needed which does not require such complicated post-ribbon fabrication processes and it is one aim of the present invention to provide such a marker material with high magnetomechanical performance without causing “pollution problem” mentioned above. Fully utilizing the new magnetomechanical marker material of the present invention, the present invention includes a marker with encoding and decoding capability and an electronic identification system utilizing the marker. A coded surveillance system having a magnetomechanical marker was taught in U.S. Pat. No. 4,510,490, but the number of constituent marker strips was limited due to a limited space available in a marker, thus limiting the universe of encoding and decoding capability using such a marker.
Clearly, a marker is needed in which the number of marker strips is increased considerably without sacrificing the performance as a coded marker in an electronic article identification system having encoding and decoding capability, hereinafter termed “coded electronic article identification system.”